The properties of liquid-crystalline materials whereby they significantly vary their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color, under the influence of electric fields, are widely utilized for electro-optical display elements. Thus, the functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the industrial application of these effects in electronic components, liquid-crystalline dielectrics are required which must meet a large number of demands. Chemical resistance to moisture, air and physical influences, such as heat, radiation in the infra-red, visible and ultraviolet regions and continuous and alternating electric fields, is of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least 0.degree. C. to +50.degree. C., preferably from -10.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 50.times.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the visible region, i.e., they must be colorless.
A number of liquid-crystalline compounds have already been disclosed, which fulfill the stability demands made on dielectrics for electronic components, and which are also colorless. However, no single compounds have yet been disclosed which fulfill all the requirements in respect of the range of temperature of the liquid-crystalline mesophase, the dielectric anisotropy, the optical anisotropy, the viscosity, the specific resistance and the shape of the electrooptical characteristic curve.
For this reason, mixtures are employed. Their compositions are adapted to the requirements in each case. In order to vary the properties of the mixtures, as many different substances as possible are required, if possible from different classes of substances. This provides sufficient scope in which to change the properties of the mixtures of substances. For this reason, there is a constant search for new liquid-crystals with advantageous properties.